Sub-assembly for dust suppression in rock drilling

ABSTRACT

This invention concerns an accessory, to be inserted in a drill string behind a drill bit, to provide separation of the liquid and air components of a flushing medium. The accessory comprises an inner housing (36) within an outer housing (34). A liquid/gas medium enters through opening (50) onto spiral raceway (48). Holes are provided at intervals along spiral (42). Liquid in the medium is centrifuged by the spiral (42) and projected into the chamber between inner and outer housings (36,34). The expelled liquid (52) falls to the base (70) of the housing (34) where it passes through apertures (38) into the drill bore. Once the medium has traversed the spiral (42) essentially only gas remains to pass through the opening (62) to the drill bit.

TECHNICAL FIELD

The present invention relates to dust suppression in rock drillingoperations and in particular to a subassembly to separate the liquidfrom the gas of the flushing medium used in rock drills.

Drilling into bedrock for example to enable explosive charges to beplaced for excavating ore in open-cut mining operations is carried outby rotary air blast drills. Air at high pressure (typically 40 psi) andvolume (750 to 2000 cubic feet a minute (cfm)) is delivered through abore in the drill string to the drill bit. The air supplied to the drillbit, which may for example be a blade or roller type bit, exits fromorifices in the bit, cools the bearings of the bit and conveys thedebris created by the drilling away from the drilling workface up theborehole. This debris travels up the borehole at a typical (bailing)velocity of 5,000 to 7,000 feet per minute depending on the size of theborehole and the drill string.

BACKGROUND OF THE INVENTION

The debris produced includes particulate matter and dust. To reduce thedispersion of dust into the environment, which has deleterious effectson both equipment and personnel, the debris is sprayed with water. Thewater is supplied with the air through the drill string to the drill bitand in addition to suppressing dust also causes corrosion, in particularof the bearings. It also produces a slurry which causes wear of thecutting surfaces of the drill bit, reducing the life of the bit andreducing drill penetration rates. The drill bit is a costly item toreplace.

A stabiliser may be located behind the drill bit to centre the drill bitwithin the borehole. The sections of the drill string are joinedtogether by threaded connectors at either end. Two standard threads aregenerally used in the drilling industry, namely BECO and API, and acrossover connection is often required to accommodate assemblies havingthese different threads.

DISCLOSURE OF THE INVENTION

According to a first aspect of the invention, there is provided a dustsuppression sub-assembly for a rock drill. The sub-assembly is providedwith means to secure it into a drill string adjacent the drill bit. Thesubassembly includes an outer housing and an inner housing, and ahelical path extends through the inner housing. Inlet means for a gasand liquid flushing medium are provided at one end of the sub-assemblycommunicating with the helical path. A plurality of slots are spaced atintervals along the inner housing communicating between the interior ofthe inner housing and a chamber defined between the inner and outerhousings. The liquid part of the flushing medium is centrifuged by itshelical motion along the path and exits the slots. A plurality ofoutlets are provided for the liquid in the outer housing adjacent theother end of the sub-assembly, and exit means are provided for the gasat the other end of the inner housing in communication with the helicalpath.

The sub-assembly can be fitted behind the drill bit or behind thestabiliser of the drill string of a rock drill. The sub-assembly isattached at either end by hollow threaded connections; the connectionsmay be either male or female.

Preferably, the helical path is formed by a spiral flange. Alternativelythe helical path is formed by a plurality of entwined tubes.

Preferably the slots are 180° apart along the helical path.

Preferably the inner and outer housings are co-axial cylinders ofdifferent radii.

Preferably scrapers are provided on the interior of the inner housing atthe trailing edges of the slots, in the direction of motion of themedium along the spiral path, to direct the liquid radially outwardsthrough the slots. Optionally the scrapers comprise curved extensions ofthe trailing edges of the slots.

Preferably the sub-assembly further comprises a return path extendingbetween a balance hole adjacent the other end of the sub-assembly, andprovided with a one-way check valve; and a return port adjacent theinlet means.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described withrespect to the following figures in which:

FIG. 1 is a partial longitudinal section of an embodiment of theinvention; and

FIG. 2 is a schematic cross section of the embodiment of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The sub-assembly comprises an outer housing 34 and an inner housing 36.The inner housing 36 is coaxial with the outer housing 34 and of lesserradius.

Around the periphery of the outer housing 34 and towards that end 40which will be located in use nearest the drill bit are a series of holes38.

The inner housing 36 has a tight-fitting internal spiral flange 42extending along its length and terminating at its lower end in a base44. The inner housing 36 has slots 46 in its wall located 180° apartalong the length of the spiral 42, and located adjacent the spiral 42.

Water and air enter at the top 48 of the spiral, through opening 50 fromthe drill string, and flow along the spiral 42. The water, as shown byarrows 52, is forced radially outwards by the centrifugal action of itsmotion along the spiral, and exits through the slots 46 into the chamber54 between the inner housing 36 and the inner wall 56 of the outerhousing 34. The water is progressively removed from the medium as itprogresses along the spiral 42.

The edges of slots 46 are shaped (see FIG. 2) with the trailing edges58, in the direction of motion of the water 52, curved inwards to act asdeflectors, or scrapers, to scrape the water from the inner walls ofinner housing 36; they direct the water 52 radially outwards.

Substantially only air 60 exits the inner housing 36 through theaperture 62 in the base 44 of spiral 42, and continues down into a base75 in the drill string to the bit.

The water 52 exiting 64 from the slots 46 continues to move outwardsunder the effect of centrifugal forces, which results in a furtherseparation of the water and any entrained air.

The water gathers 66 at the inner surface 56 of the outer housing 34,and the air gathers towards the outer surface 68 of the inner housing36. The water separated by this action travels down the inner wall 56 ofthe outer housing 34 to exit from the assembly through holes 38 at thebases of taper boss 70. These holes 38 allow water to spray the debrisforced up the borehole by pressurised air exiting from the drill bit ina manner to be described more fully below. The pressure in the spacebetween the inner and outer housing is positive with respect to theoutside of the outer housing (the borehole) to drive the water throughthe holes 38. The size and number of holes 38 depends on the volume ofwater to be dispersed and on the desire to avoid excessive pressureloss.

The air gathered near the outer wall 68 of the inner housing 36 canre-enter the inner housing 36 through slots 46.

A balance hole 72 is provided near the base of the inner housing 36. Thebalance hole 72 leads into the central pillar 74 of the spiral via a oneway check valve 73. Excess water and air are returned through centralpillar 74 to a return port 75 adjacent the top 48 of the spiral.

The debris created by the action of the drill bit is driven upward fromthe drill bit up the borehole by the air and upon reaching the region ofthe holes 38 in the outer housing 34 becomes wet, the air then carriesthe resultant slurry further up the borehole.

Little air remains in the water 52 compared to that which was initiallyfed in at the top 48 of the spiral.

Accordingly, essentially dry air (90% to 98% air) is delivered to thedrill bit. The region of the drill bit is therefore subject to the blastof air only, rather than to the generation of a thick slurry of water,air and debris which rapidly wears the drill bit, and its bearings. Thesubstantial absence of water from the drill bit reduces water corrosionof the bearings.

The sub-assembly can be connected to the drill bit or to the stabiliser,which is generally employed to centre the drill bit in the borehole. Itcould comprise a separate sub-assembly or could be built into thestabiliser. More than one unit may be employed to produce the requiredseparation of the water from the flushing medium.

Although the invention has been described with reference to a particularembodiment it should be appreciated that it may be embodied in manyother ways. For instance, the number of turns and pitch of the spiral 42and the number of slots 46 will depend on the volume of water and airpassing through the drill string. The size of the drill string and henceof the housings 34, 36 will depend on the application. It is preferredthat the cross sectional area of the spiral 42 is greater than that ofthe inlet 50 to reduce any adverse effects that may be created by backpressure developing in the inlet.

It is understood that turbulence in the flow is to be avoided to alloweasier separation of the water by the scraping action of the slots 46.If the flow is not substantially laminar then extra turns of the spiral42 may be required or sub-assemblies in tandem may be employed.

In an alternative embodiment the inner housing and spiral flange may bereplaaced by a number of tubes entwined like braid or rope into a helix.Holes at intervals along each of the tubes allow water to escape underthe centrifugal action of its circulation through the tubes in a similarmanner to that described above. The outer housing would be as describedfor the above embodiment.

We claim:
 1. A dust suppression sub-assembly for a rock drillcomprising:means to secure the sub-assembly into a drill string; anouter housing and an inner housing; a helical path extending through theinterior of the inner housing; inlet means for a gas and liquid flushingmedium at one end of the sub-assembly, communicating with the helicalpath; a plurality of slots at intervals along the inner housingcommunicating between the interior of the inner housing and a chamberdefined between the inner and outer housings; a plurality of outlets forthe liquid in the outer housing adjacent the other end of thesub-assembly; and exit means for the gas at the other end of the innerhousing communicating with the helical path.
 2. A dust suppressionsub-assembly according to claim 1 wherein the helical path is formed bya spiral flange.
 3. A dust suppression sub-assembly according to claim 1wherein the helical path is formed by a plurality of entwined tubes. 4.A dust suppression sub-assembly according to claim 1 wherein the slotsare 180° apart along the helical path.
 5. A dust suppressionsub-assembly according to claim 1 wherein the inner and outer housingsare co-axial cylinders of different radii.
 6. A dust suppressionsub-assembly according to claim 1 wherein scrapers are provided on theinterior of the inner housing at the trailing edges of the slots, in thedirection of motion of the medium along the spiral path, to direct theliquid radially outwards through the slots.
 7. A dust suppressionsub-assembly according to claim 6 wherein the scrapers comprise curvedextensions of the trailing edges of the slots.
 8. A dust suppressionsub-assembly according to claim 1, further including a return pathextending between a balance hole adjacent the other end of thesub-assembly, and provided with a one-way check valve, and a return portadjacent the inlet means.